The present invention relates to a process for polymerizing vinyl aromatic compounds. More particularly, the present invention relates to a process for polymerizing in a mold, i.e., reaction injection molding a fluid vinyl aromatic compound, optionally containing additional components such as crosslinking agents, reinforcing agents, prepolymer and impact modifiers.
It has been previously known to polymerize a vinyl aromatic compound in a mold in order to directly prepare molded compositions. The process is rendered particularly difficult by the fact that pure styrene, when allowed to polymerize adibatically, may achieve excessive temperatures. For example, a temperature rise to an excess of 300.degree. C. is possible. This extreme temperature would be above or at least near the decomposition point of polystyrene. Accordingly, it is necessary in the polymerization of styrene monomer in a mold to resort to methods in order to moderate the release of heat due to polymerization. Suitable means for moderating the increase of temperature include the use of inert diluents in sufficient quantity to moderate the reaction, for example, by absorbing reaction heat via evaporation under reflux. However, in a polymerization in-mold process, the presence of inert diluents may be disadvantageous. The use of such a diluent introduces operational difficulties including the problem of removing residual diluent from the finished polymerized product.
A further disadvantage with in-mold polymerization is caused by the increase in density between polymerized and unpolymerized material resulting in shrinkage of formed parts.
In GB No. 1,174,845, there is disclosed a process employing the use of at least 10 percent by weight of a substantially non-volatile additive which is inert with respect to the polymerization in order that at least a major proportion of the exothermic heat of polymerization may be absorbed within the polymerization mixture. More particularly, the additive employed in the abovedescribed reference includes a polymer having a molecular weight appropriate to the need to dissolve the additive in the vinyl aromatic compound prior to polymerization thereof. Preferably, such polymers were described as polymers of the compound to be polymerized having a weight average molecular weight within the range of 500 to 30,000.
J. D. Muzzy et al. disclosed that in-mold polymerization of styrene using free radical initiators may be achieved in very short reaction times, on the order of about 15 minutes. Crosslinking comonomers were added in order to reduce the amount of unreacted monomer remaining in the product. Fillers, especially prepolymer of the vinyl aromatic compound to be polymerized and foaming agents were also incorporated in order to compensate for the previously discussed polymerization shrinkage and to accommodate the reaction exotherm. See, 1980 ANTEC Meeting, Society of Plastics Engineers, page 638; J. D. Muzzy et al., Preprints of Organic Coating and Plastics Materials, Volume 44, ACS Meeting, Atlanta, March, 1981, page 145.
Previously known processes for preparation of in-mold polymerization products have not proven commercially adaptable. The major problem has continued to be an inability to control the effects of shrinkage while at the same time maintaining adequate control of the heat generated by the polymerizing reaction mixture. Uncontrolled shrinkage results in failure of the molded article to conform adequately to the mold contours resulting in rejected molding parts. Polymerization of a fluid monomer or monomer solution within a mold offers attractive economic advantages over injection molding of a corresponding thermoplastic object. The economic advantage stems from the use of less energy due to the fact that polymerization is accomplished at the time of molding, thereby avoiding the necessity of remelting a thermoplastic polymer prior to injection to a mold. In addition, reduced viscosity of monomers as compared to melted thermoplastics leads to the use of lower pressure processes, especially for the molding of larger parts, thereby resulting in lower energy costs, and the use of molding equipment and molds which need not be built to withstand elevated pressures.
It would be desirable to provide a process for polymerizing a polymerizable reaction mixture in a mold wherein the polymerization mixture is reasonably fluid at or near ambient temperatures. In addition, it would be desirable to provide a process for polymerization in a mold of a reaction mixture whereby the mold may be filled with the polymerization mixture at relatively low pressure. In addition, it would be desirable to provide a polymerization process wherein the exothermic heat of reaction of the polymerization process does not lead to thermal decomposition or excessive pressure within the mold. In addition, it would be desirable to provide a process for polymerization in a mold of a molded object which provides better conformation with the contours of the mold and results in fewer surface defects or cavities in the molded object. In addition, it would be desirable to provide a process for in-mold polymerization of a molded object having improved physical and mechanical properties.
It is the accomplishment of these objects to which the present invention is addressed.